Many types of data cartridges exist, such as the two-hub, belt-driven magnetic tape data cartridge as shown in FIG. 1. The typical data cartridge has a housing which encloses a pair of rotatably mounted tape hubs about which tape is wound. In the course of passing the tape from one hub to the other, the tape passes along a predefined tape path in the data cartridge which moves the tape past a cut away portion through which a head of a drive can access the tape. The data cartridge housing also typically encloses various rollers, such as, for example, comer rollers and a drive roller. A drive belt is stretched around the rollers, including the drive roller, and the tape packs on the hubs. The drive roller is contacted by a drive mechanism of a tape drive through an opening formed at the front of the housing. Under control of a processing unit of the tape drive, movement of the drive roller by the drive mechanism causes the drive belt to move and frictional engagement between the drive belt and the tape then causes the tape to move. In addition to moving the tape, the drive belt also provides for tension in the tape. The tape can be moved and read in either direction, i.e., either hub can serve as the supply or as the take-up hub depending on the direction of the movement of the tape.
Tape is wound into the data cartridge resulting in a tape pack with a certain tape and belt tension. These tensions generate forces in the tape pack to alleviate tape pack shift and problems that may result therefrom. However, when a data cartridge is subjected to temperature changes, the forces holding the tape in position in the tape pack change. These forces may go below a certain level at some position of the tape pack causing the pack to shift. In other words, at least a portion of the tape wound within the tape pack loses tape tension and either shifts towards, for example, the cover or the base plate of a data cartridge.
There are numerous environmental circumstances that may lead to such tape pack shift. For example, a mechanical shock to the data cartridge may produce tape pack shift, as well as the above mentioned temperature changes.
Further, short shuffling of the tape, i.e., operation or movement of the tape from one hub toward the other hub repetitively for less than the full length of the tape, may also result in tape pack shift or changes in the forces that hold the tape in place. In other words, when the tape is moved from the beginning of the tape (BOT) towards the end of the tape (EOT) without going all the way to the EOT, a point of turn around occurs somewhere between the BOT and EOT. At the point of turn around, the pack may lose tension causing tape pack shift.
Various degrees of tape pack shift may occur. For example, the amount of shift due to short shuffling over a short period of time may be much less than the shift due to exposure of the cartridge to an extreme temperature change. In some circumstances, if a shifted tape pack is subjected to sudden acceleration or deceleration, tape pack scramble may occur, i.e., the tape may lose its normal wound position or its position on the guide pins causing catastrophic failure of the data cartridge.
Conventionally, tape pack shift is removed by retensioning of the tape, i.e., tape pass from BOT to EOT and back to BOT, each time a data cartridge is inserted into a tape drive (which may hereinafter be referred to as automatic retensioning). For example, if such data cartridges are utilized for operation in a library system, each time the data cartridge is inserted into the tape drive, the tape pack of the data cartridge must be retensioned. This may require a time period of more than four minutes every time the tape is retensioned. Such time delays for a user attempting to access information on the data cartridge is undesirable.
For the above reasons and other reasons that will become apparent from the description below, alternatives to conventional retensioning of tape packs to avoid failures of tape pack shifted data cartridges are needed. For example, any such alternative must overcome the need to retension the tape pack of a data cartridge at every insertion into the tape drive.